A loft conversion is one of the most valuable home improvements you can make — and one of the few where the planning rules are genuinely generous. Most loft conversions are permitted development. But the front dormer trap and the volume limits catch a significant number of projects, and the consequences of getting it wrong on a £40,000 build are serious.
Rear dormers: usually permitted development. Front dormers: almost always need planning permission. Hip-to-gable conversions: always need planning permission.
A loft conversion is permitted development under Class B of Part 1 of the GPDO if it is on the rear or side roof slope not visible from a highway at the front, the volume added does not exceed 40 cubic metres (terraced houses) or 50 cubic metres (detached and semi-detached), it does not extend beyond the highest point of the existing roof, no verandas or balconies are added, side-facing windows are obscure-glazed and non-opening below 1.7m, and the materials are similar in appearance to the existing house.
This is the single most common loft conversion planning mistake. A dormer on the front of the house — facing the street — is not permitted development. The rule is clear: no front dormers under permitted development. Many builders will advise otherwise.
Converting a hip roof to a gable roof changes the fundamental shape of the building and is never permitted development regardless of size. The 40/50 cubic metre limit is also cumulative — if a previous owner already added a rear dormer, that volume counts against your allowance. The planning history shows this.
Side-facing windows in a loft conversion must be obscure-glazed and non-opening below 1.7 metres above floor level. A non-compliant window means the conversion is not permitted development. In a conservation area, adding a rooflight on a front-facing slope is not permitted development.
Standard searches check the public register. We go further — querying live portals, blocked legacy systems, pre-merger authority databases, committee PDF archives, Land Registry title constraints, and comparable decisions across your postcode cluster. What we retrieve determines what you know before you build, buy, or appeal.